Electronic parts are incorporated in an apparatus with use of a film carrier tape constituted of a flexible insulating film and a number of wiring patterns formed thereon.
Such film carrier tapes for mounting electronic parts include TAB (tape automated bonding) tapes, CSP (chip size package) tapes, COF (chip on film) tapes, BGA (ball grid array) tapes, ASIC (application specific integrated circuit) tapes, two-metal (double-sided) tapes and tapes for multilayer interconnection. These film carrier tapes have inner leads for the connection with terminals of electronic parts, and outer leads, which are in series with respective inner leads, for establishing connection with outside devices.
Depending on the type of such film carrier tapes, it is often required to plate the outer leads with solder, an alloy of tin and lead. Since the solder contains lead which is a heavy metal, it has been replaced with a tin-bismuth alloy due to the recent global request for elimination of the lead metal. Plating with the solder or tin-bismuth alloy is generally performed on outer leads of film carriers that are provided in the vicinity of the edges in the width direction of the long film carrier tape. To accomplish such selective plating on the outer leads near the edges in the width direction of the film carrier tape, the film carrier tape is fed standing perpendicular to the surface of the plating solution so that an area near the downside edge in the width direction thereof will be immersed in the plating solution. As a consequence, the wiring patterns (outer leads) found in the immersed area can be selectively plated in the plating solution. This selective plating method is generally called the “plating by liquid level control”. One operation of the plating by liquid level control results in formation of deposits of, for example, the tin-bismuth alloy on the outer leads in the vicinity of one tape edge in the width direction. Another operation for the reverse edge of the film carrier tape produces the same effect on the other side. Accordingly, the outer leads in both edge areas in the width direction of the film carrier tape can be plated with the tin-bismuth alloy. On the other hand, in the plating by liquid level control, inner leads found in the center area in the width direction of the film carrier tape remain unplated with the tin-bismuth alloy.
In the above plating by liquid level control, electroplating is generally employed to form deposits of the tin-bismuth alloy. Specifically, the electroplating is performed in a plating tank filled with a tin-bismuth alloy plating solution in a manner such that the desired portion of the film carrier tape for mounting electronic parts is immersed in the plating solution and an electric current is applied using the wiring patterns (bus conductor leads) of the film carrier tape as cathodes and insoluble electrodes of platinum or the like as anodes.
When a voltage is applied between the electrodes to carry out the electroplating, various electrochemical reactions take place in the plating solution, causing generation of gas from the anodes and cathodes. For example, the following reactions occur in the anodes and cathodes:
Anodes: 2OH−→½O2+H2O+2e−
Cathodes: Sn2++2e−→Sn                Bi3++3e−→Bi        
When the current density is high, the reaction H++e−→½H2 also occurs in the cathodes.
It is often the case that the plating solution contains a surfactant or the like for enhancing affinity for the object to be plated. The gas generated during the electroplating eventually forms bubbles on the surface of the plating solution. When the plating solution contains a surfactant in particular, these bubbles are more resistant to breakage and survive relatively longer floating on the plating solution.
Meanwhile, it is conventional practice in the production of film carrier tapes for mounting electronic parts that a film carrier tape wound on a reel is unwound and fed into and through, for example, a plating tank in which the desired metal is deposited on the tape. When the film carrier tape travels in the plating tank, the plating solution also is caused to flow in the traveling direction of the tape. The flow of the plating solution carries the bubbles formed, occasionally bringing some bubbles to adhesion to the traveling film carrier tape.
In the plating by liquid level control, the outer leads to be plated generally have width of several tens of μm, which is the level that will lead to plating failure by any slight variation in the liquid level of plating solution due to the adhered bubbles.
The present invention has an object of providing a plating machine for plating a film carrier tape for mounting electronic parts that can reduce the probability that bubbles of gas which are vigorously generated in a plating solution during electroplating will float in the vicinity of wiring patterns and attach to the wiring patterns.
It is another object of the present invention to provide a process for producing film carrier tapes for mounting electronic parts that allows for the formation of deposits with minimum adverse effects of bubbles even if such bubbles are being vigorously generated in the plating solution during electroplating.